1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium and a magnetic storage apparatus using the perpendicular magnetic recording medium.
2. Description of the Related Art
Recently, application and importance of magnetic storage apparatuses, such as a magnetic disk drive, a flexible disk drive, and a magnetic tape drive, are increasing, and recording densities of magnetic recording media used in such magnetic storage apparatuses have improved considerably. Particularly in an HDD (Hard Disk Drive) to which an MR (Magneto-Resistive) head and a PRML (Partial Response Maximum Likelihood) technique have been applied, a surface recording density has increased considerably. Recently, with the use of GMR (Giant Magneto-Resistive) head, a TuMR (Tunneling Magneto-Resistive) head, or the like, the surface recording density continues to increase at a rate of approximately 1.5 times per year.
On the other hand, a magnetic recording system used in the HDD is conventionally the in-plane magnetic recording system, but is recently being replaced by the so-called perpendicular magnetic recording system. The perpendicular magnetic recording system is suited for increasing the surface recording density, because crystal grains of a magnetic recording layer for recording information have an axis of easy magnetization aligned in a direction perpendicular to a substrate surface.
A perpendicular magnetic recording medium generally includes a nonmagnetic substrate, a backing layer made of a soft magnetic material, an underlayer, and a perpendicular magnetic recording layer, where the underlayer controls a perpendicular orientation of the perpendicular magnetic recording layer.
However, with the increase in the surface recording density, there is a limit to recording and reproducing information “magnetically (or by magnetization)” according to the conventional perpendicular magnetic recording system.
In order to record at a high density, an area on the magnetic recording medium occupied by one bit (that is, a minimum unit of data to be magnetically recorded) needs to be reduced. However, the grain diameter or magnetic cluster in the magnetic recording layer of the perpendicular magnetic recording medium needs to be reduced, in order to reduce the area occupied by one bit. This is because, if the grain diameter or magnetic cluster in the magnetic recording layer is maintained at the existing size, an SNR (Signal-to-Noise Ratio) of data that are recorded and reproduced deteriorates, and a sufficiently high recording and reproducing characteristics cannot be obtained from the perpendicular magnetic recording medium.
When the grain diameter of magnetic cluster is reduced, it is known that stability (that is, thermal stability) of recorded data deteriorates due to the effects of heat, and the so-called heat fluctuation occurs. A magnetic material having a high crystal magnetic anisotropy constant Ku (hereinafter also referred to as a “high-Ku material”) must be used for the magnetic recording layer in order to maintain the thermal stability. However, when the high-Ku material is used, the coercivity of the magnetic recording layer increases, thus requiring a high recording magnetic field in order to record the data.
On the other hand, the recording magnetic field of the magnetic head is highly dependent on the magnetic material used for a recording element (or magnetic pole). For this reason, there is a limit to the recording magnetic field that can be generated by the magnetic head.
In order to suppress the above described problem, there are proposals to record the data on the magnetic recording layer using an electric field. For example, Japanese Laid-Open Patent Publication No. 2006-139854 proposes generating the electric field on the side of the magnetic recording medium at the time of the recording using magnetization, in order to reduce the coercivity of the magnetic recording layer. On the other hand, Japanese Laid-Open Patent Publication No. 2007-265512 proposes an MAMR (Microwave Assisted Magnetic Recording) utilizing resonance between a high-frequency electric field and a magnetic spin of the magnetic recording medium, in order to reduce the coercivity of the magnetic recording layer. The first proposal requires the side of the magnetic recording medium to be grounded within the magnetic storage apparatus, and the configuration of the head to simultaneously generate and control the magnetic field and the electric field becomes complex. On the other hand, the second proposal requires a mechanism to generate the high-frequency electric field, and the configuration of the head having such a mechanism built therein becomes complex.
For example, Japanese Laid-Open Patent Publication No. 2008-219007 proposes a system using a ferroelectric recording layer, and recording and reproducing the data using an electric field rather than a magnetic field. According to this proposed system, a voltage simply needs to be applied when recording the data on the magnetic recording medium. In addition, because the data is reproduced using an electric field, the recording and reproducing element can be formed, not by a magnetic material, but by a conductive probe and a coil that generates the magnetic field, and the limit to the recording capability may be increased. However, this proposed system has unstable polarization that locally exists, and slow recording speed, when compared to the conventional magnetic recording system.
Recently, there is active research on materials having both magnetic and ferroelectric properties. Attention is particularly drawn to multiferroic materials having both magnetic and ferroelectric properties. Examples of known multiferroic materials displaying both magnetic ferroelectric properties include BiFeO3, and materials having Bi substituted by Ba, and Fe substituted by Mn. By incorporating such materials in the conventional magnetic recording layer made of the magnetic material, it becomes possible to record data using an electric field, and to reproduce data using a magnetic field, as reported in Yusuke Sugawara et al., “Low-temperature fabrication of BiFeO3 based multiferroic thin films with (111) orientation by sputtering deposition process with VHF plasma irradiation”, Proceedings of the 60th Symposium of Applied Physics Society Spring Scientific Presentation, 28a-D3-8, pp. 6-17, 2013, for example.
The recording with respect to the magnetic recording medium using the material having both magnetic and ferroelectric properties can be performed by an electric field. Hence, by reducing the size of the recording element that generates the electric field, a high recording density can be achieved. On the other hand, the materials displaying both magnetic and ferroelectric properties in many cases take the form of a perovskite structure including oxygen. Such a perovskite structure is a complex structure, and crystallinity plays an important role in developing such properties. In addition, a composition of such a perovskite structure may easily deviate from stoichiometry of the composition and cause oxygen deficiency or the like. Furthermore, according to studies made by the present inventors, when the crystallinity of such materials displaying both magnetic and ferroelectric properties is improved, the size of the crystal grains increase considerably. For this reason, it is difficult to form a smooth medium surface using such materials, and makes the floating of the head unstable. Moreover, structural defects caused by the oxygen deficiency and the increased size of the crystal grains generate magnetic noise when the magnetic head is used to reproduce information from the magnetic recording medium.